Serially Connected Inverters

ABSTRACT

A photovoltaic power generation system, having a photovoltaic panel, which has a direct current (DC) output and a micro-inverter with input terminals and output terminals. The input terminals are adapted for connection to the DC output. The micro-inverter is configured for converting an input DC power received at the input terminals to an output alternating current (AC) power at the output terminals. A bypass current path between the output terminals may be adapted for passing current produced externally to the micro-inverter. The micro-inverter is configured to output an alternating current voltage significantly less than a grid voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to patent applicationGB1100450.4, filed Jan. 12, 2011, in the United Kingdom IntellectualProperty Office. Application GB1100450.4 is herein incorporated byreference

FIELD OF THE INVENTION

Aspects generally relate to distributed power system and moreparticularly to the use of multiple micro-inverters.

BACKGROUND

Recent increased interest in renewable energy has led to research anddevelopment of distributed power generation systems includingphotovoltaic cells and fuel cells. Various topologies have been proposedfor connecting these power sources to the load, taking intoconsideration various parameters, such as voltage/current requirements,operating conditions, reliability, safety, costs. These sources providelow voltage direct current output (normally below 3 Volts), so they areconnected serially to achieve the required voltage. Conversely, a serialconnection may fail to provide the required current, so that severalstrings of serial connections may be connected in parallel to providethe required current.

Power generation from each of these sources typically depends onmanufacturing, operating, and environmental conditions of the powersources, e.g. photovoltaic panels. For example, various inconsistenciesin manufacturing may cause two identical sources to provide differentoutput characteristics. Similarly, two identical sources may reactdifferently to operating and/or environmental conditions, such as load,temperature, etc. In practical installations, different source may alsoexperience different environmental conditions, e.g. in solar powerinstallations some panels may be exposed to full sun, while others beshaded, thereby delivering different power output.

Islanding is a condition where a power generation system is severed fromthe utility network, but continues to supply power to portions of theutility network after the utility power supply is disconnected fromthose portions of the network. Photovoltaic systems must haveanti-islanding detection in order to comply with safety regulations.Otherwise, the photovoltaic installation may electrically shock orelectrocute repairpersons after the grid is shut down from thephotovoltaic installation generating power as an island downstream. Theisland condition poses a hazard also to equipment. Thus, it is importantfor an island condition to be detected and eliminated.

The process of connecting an alternating current (AC) generator or powersource (e.g. alternator, inverter) to other AC power sources or thepower grid is known as synchronization and is crucial for the generationof AC electrical power. There are five conditions that are met for thesynchronization process. The power source must have equal line voltage,frequency, phase sequence, phase angle, and waveform to that of thepower grid. Typically, synchronization is performed and controlled withthe aid of synch relays and micro-electronic systems.

The term “grid voltage” as used herein is the voltage of the electricalpower grid usually 110V or 220V at 60 Hz or 220V at 50 Hz.

BRIEF SUMMARY

According to various aspects there is provided a micro-inverter havinginput terminals and output terminals. The micro-inverter may be adaptedfor inverting an input DC power received at the input terminals to anoutput alternating current (AC) power at the output terminals, whichhave a voltage significantly less than a grid voltage. A bypass currentpath between the output terminals may be adapted for passing currentproduced externally to the micro-inverter. An optional synchronizationmodule may be adapted for synchronizing the output AC power to the gridvoltage. A control loop may be configured to set the input DC powerreceived at the input terminals according to a previously determinedcriterion. The previously determined criterion typically sets a maximuminput power.

According to various aspects there is provided a photovoltaic powergeneration system having multiple photovoltaic panels with directcurrent (DC) outputs connectible to multiple micro-inverters. Eachmicro-inverter has input terminals connectible to the DC outputs andoutput terminals. The micro-inverters are configured for inverting inputDC power received at the input terminals to an output alternatingcurrent (AC) at the output terminals with an output voltagesubstantially less than a grid voltage. The output terminals areconnectible in series into a serial string and an output voltage of theserial string may be substantially equal to the grid voltage. Eachmicro-inverter includes a bypass current path between the outputterminals for passing current produced externally in the serial string.The alternating current (AC) micro-inverter may have a control loopconfigured to set the input DC power received at the input terminalsaccording to a previously determined criterion. An optional centralcontrol unit may be operatively attached to the serial string and thegrid voltage. The central control unit may be adapted for disconnectingthe system from the grid upon detecting a less than minimal gridvoltage. The central control unit optionally monitors thesynchronization of the voltage of the serial string to the grid voltageand disconnects the serially connected micro-inverters from the grid ordisables the micro-inverters upon a lack of synchronization between thegrid voltage and the output voltage of the serially connectedmicro-inverters.

According to various aspects there is provided a method for photovoltaicpower generation in a system having multiple of photovoltaic panels withdirect current (DC) outputs and multiple micro-inverters each includinginput terminals and output terminals. The input terminals of themicro-inverters are connectible to respective DC outputs of thephotovoltaic panels. The output terminals are connected serially to aserial voltage output. The DC power received at the input terminals maybe inverted to an output alternating current (AC) power at the outputterminals while maintaining the serial voltage output substantiallyequal to a grid voltage. The output terminals preferably have a currentbypass in the event of failure of inverting the DC power received at theinput terminals to the output alternating current (AC) power at theoutput terminals or upon the micro-inverter being shut down in the eventof a failure to maintain the serial voltage output at the level of thegrid voltage.

Upon connecting the input terminals and the output terminals, inversionof input DC power to output power may be enabled after a previouslydetermined time delay. The serial voltage output may be synchronized tothe grid voltage. The output terminals preferably have a current bypassin the event of failure of inverting the DC power received at the inputterminals to the output alternating current (AC) power at the outputterminals or upon the micro-inverter being shut down in the event of afailure to maintain the serial voltage output at the level of the gridvoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 shows a conventional installation of a solar power system.

FIG. 2 illustrates one serial string of DC sources.

FIG. 3 illustrates a power harvesting system.

FIG. 4a illustrates a power harvesting system in accordance with one ormore embodiments of the disclosure.

FIG. 4b illustrates a power harvesting system in accordance with one ormore embodiments of the disclosure.

FIG. 4c illustrates further details of a bypass in accordance with oneor more embodiments of the disclosure.

FIG. 5a illustrates a method of operation of a power harvesting systemin accordance with one or more embodiments of the disclosure.

FIG. 5b shows further details of connection and wake-up of a powerharvesting system in accordance with one or more embodiments of thedisclosure.

FIG. 5c shows further details of operation in accordance with one ormore embodiments of the disclosure.

The foregoing and/or other aspects will become apparent from thefollowing detailed description when considered in conjunction with theaccompanying drawing figures.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments, examples ofwhich are illustrated in the accompanying drawings, wherein likereference numerals refer to the like elements throughout. Variousaspects are described below with reference to the figures.

A conventional installation of a solar power system 10 is illustrated inFIG. 1. Since the voltage provided by each individual photovoltaic panel100 is low, several panels 100 are connected in series to form a string102 of panels 100. For a large installation, in order to achieve highercurrent, several strings 102 may be connected in parallel. Photovoltaicpanels 100 are mounted outdoors, and are connected to a maximum powerpoint tracking (MPPT) module 106 and to an inverter 104. MPPT 106 istypically implemented in the same housing as inverter 104.

Harvested power from the DC sources is delivered to inverter 104, whichconverts the fluctuating direct-current (DC) into alternating-current(AC) having a desired voltage and frequency, which, for residentialapplication, is usually 110V or 220V at 60 Hz or 220V at 50 Hz. ACcurrent from inverter 104 may then be used for operating electricappliances or fed to the power grid. Alternatively, if the installationis not tied to the grid, the power extracted from inverter 104 may bedirected to store the excess power in batteries.

FIG. 2 illustrates one serial string of DC sources according toconventional art, photovoltaic panels 100, connected to MPPT circuit 106and inverter 104 to form a power harvesting system 20 connected to load108. The current versus voltage (IV) characteristics are plotted to theleft of each photovoltaic panel 100. For each photovoltaic panel 100,the current decreases as the output voltage increases. At some voltagevalue the current goes to zero, and in some applications may assume anegative value, meaning that some photovoltaic panels 100 instead ofbeing sources of power become sinks of power. Bypass diodes (not shown)connected in parallel across each photovoltaic panel 100 output are usedto prevent any photovoltaic panel 100 from becoming a sink of power. Thepower output of each photovoltaic panel 100 is equal to the product ofcurrent and voltage (P=I*V) and varies depending on the voltage drawnfrom the panel 100. At a certain current and voltage, the power reachesits maximum (represented by the dot on the IV curve for each graph). Itis desirable to operate a panel 100 at this maximum power point (MPP).The purpose of the maximum power point tracking (MPPT) module 106 is tofind a suitable “average” maximum power point (MPP) for all panels 100.The maximum power point of the string selected by MPPT module 106 isshown using a dotted line with label MPP. The maximum power point of thestring of panels 100 is generally not the maximum power of all panels100. The dots indicating maximum power point of the individual panels100 do not fall on the dotted line marked MPP.

FIG. 3 illustrates another power harvesting system 30 according toconventional art, which combines power of multiple photovoltaic panels100. Each photovoltaic panel 100 has a direct current (DC) outputconnected to the input of an inverter 104. A bypass diode 310 isconnected in parallel across the direct current (DC) output panel 100for safety requirements. Inverter 104 receives the direct current (DC)output of photovoltaic panel 100 and converts the direct current (DC) togive an alternating current (AC) at the output of inverter 104. Maximumpower point tracking (MPPT) module 106 is typically implemented as partof the inverter 104. The outputs of multiple inverters 104 (with inputsattached to multiple photovoltaic panels 100) are connected in parallelto produce an alternating current (AC) output 304. Alternating current(AC) output 304 supplies load 108. Load 108 typically is an alternatingcurrent (AC) power grid, alternating current (AC) motor or a batterycharging circuit.

Before explaining various aspects in detail, it is to be understood thatembodiments are not limited to the details of design and the arrangementof the components set forth in the following description and illustratedin the drawings. Other embodiments are capable of being practiced andcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

By way of introduction, aspects are directed to serially connectedinverters in a grid connected photovoltaic system. In a system withserially connected inverters, as opposed to conventional system 30 whichillustrates parallel connected inverters, each inverter is required tooutput a low voltage, for instance 24 volts AC root mean square (RMS)for ten serially connected inverters. Low output voltage of themicro-inverter is suitable for efficient and low cost micro-invertertopologies. One such topology is discussed in IEEE Transactions on PowerElectronics, Vol. 22, No. 5, September 2007, entitled “A Single-StageGrid Connected Inverter Topology for Solar PV Systems With Maximum PowerPoint Tracking, this paper proposes a high performance, single-stageinverter topology for grid connected PV systems.

The term “bypass” as used herein refers to an alternate low impedancecurrent path around or through a circuit, equipment or a systemcomponent. The bypass is used to continue operation when the bypassedcircuit is inoperable or unavailable.

The terms “wake-up” and “shut-down” as used herein refer to processesduring, which a photovoltaic system is activated or de-activatedrespectively. A criterion for “wake-up”, i.e. activation of aphotovoltaic panel, for instance, is that a photovoltaic panel isexposed to sufficient light such as at dawn A criterion for “shut-down”,i.e. de-activation of a photovoltaic panel, is that a photovoltaic panelis not exposed to sufficient light, for example at dusk.

Reference is now made to FIG. 4a , which illustrates a power harvestingsystem 41 according to some embodiments. Photovoltaic inverting modules410 each have panel 100, bypass diode 310, a control loop 404 andmicro-inverter 402. Micro-inverters 402 may have optionalsynchronization units 408 and current bypass paths 422. Photovoltaicpanels 100 have direct current (DC) outputs, which are connectedrespectively to the input of inverters 402. Bypass diodes 310 mayconnected in parallel across the direct current (DC) outputs of eachpanel 100 for safety requirements (e.g. IEC61730-2 solar safetystandards). Control loops 404 are configured according to apredetermined criterion, typically to maintain maximum power at theinputs of micro-inverters 402, i.e. from the direct current (DC) outputsof photovoltaic panels 100. Bypass paths 422 are optionallynormally-closed relays, which open during operation, and which areconnected respectively to the outputs of photovoltaic inverting modules410. Photovoltaic inverting modules 410 have alternating current (AC)outputs with voltage V_(a) and current I_(a) from module 410 a; voltageV_(b) and current I_(b) from module 410 b; voltage V_(n) and currentI_(n) from module 410 n. Outputs of modules 410 are connected in seriesto give a voltage output V_(out, which) is applied to a load 406 viaswitch 414. Switch 414 is preferably controlled by control unit 418.Load 406 typically is an alternating current (AC) power grid,alternating current (AC) motor or a battery charging circuit. Controlunits 408 typically provide control signals to synchronization units 408in order to achieve synchronization with load or grid 406.Synchronization units 408 or control unit 418 provide anti-islandingfunctionality for power harvesting system 41.

Additionally, the outputs of photovoltaic inverting modules 410 a-410 nare bypassed (i.e. the output of modules 410 a-410 n are shortcircuited) by bypass 422 in the event of under voltage production bymicro inverter modules 402 or the bypass is opened (i.e. modules 410a-410 n are open circuit) in the event of over voltage by micro invertermodules 402 or during a situation of anti-islanding.

Reference is now made to FIG. 4c , which illustrates further details ofbypass 422 according to various embodiments. Bypass 422 is controlled bycontrol logic module 460, e.g. a microprocessor 460 controllingmicro-inverter 402. Microprocessor 460 has a sensing input connected tothe output voltage (V_(microinverter)) of micro inverter 402. Controllogic module 460 has other inputs connected across the bypass path atnodes A and B. Control logic module 460 has two outputs; one outputconnects to the gate of a metal oxide semi-conductor field effecttransistor (MOSFET) Q₁, the other output connects to the gate of MOSFETQ₂. The drain of MOSFET Q₁ is connected to node A and the source ofMOSFET Q₁ is connected to the source of MOSFET Q₂, the drain of MOSFETQ₂ is connected to node B. MOSFET Q₁ has a diode with an anode connectedto the drain and a cathode connected to the source. MOSFET Q₂ has adiode with an anode connected to the drain and a cathode connected tothe source. The bypass current (I_(bypass)) path is identified betweennodes A and B.

A high impedance path is provided between nodes A and B when microinverter 402 is producing an alternating current (AC) voltagesynchronized to grid voltage 406. The high impedance path is providedbetween nodes A and B when MOSFETs Q₁ and Q₂ are turned off by controllogic unit 460. When the high impedance path is provided between nodes Aand B currents I_(b), I_(X), I_(in), I_(a), I_(Y) and I_(out) are equalaccording to Kirchhoffs current law. A low impedance path is providedbetween nodes A and B when micro inverter 402 is not producing an ACvoltage and another serially-connected micro inverter 402 is producingan AC voltage. A low impedance path is provided between nodes A and B byalternately switching MOSFETs Q₁ and Q₂ on and off alternately viacontrol logic unit 406. When the load 406 is a grid voltage Q₁ and Q₂are turned alternately on and off according to the frequency of the gridvoltage. When the load 406 is a load, Q₁ and Q₂ are turned alternatelyon and off according to the frequency of synchronized inverters 402a-402 n. In the case of low impedance path being provided between nodesA and B in the embodiment according to FIG. 4a ; switching MOSFETs Q₁and Q₂ on and off by control logic unit 460 is achieved viacommunication signals between central control unit 408 and control units408 a-408 n. In the case of low impedance path being provided betweennodes A and B in the embodiment according to FIG. 4b ; switching MOSFETsQ₁ and Q₂ on and off alternately by control logic unit 460 is achievedvia communication signals between control units 408 a-408 n andinformation of grid voltage 406 via sensor 416. A low impedance pathprovided between nodes A and B means that currents I_(b), I_(bypass) andI_(out) are substantially equal according to Kirchhoff's current law. Alow impedance path provided between nodes A and B means that currentI_(bypass) flows alternately from drain to source of Q₂ and the diode ofQ₁ for one half cycle and for the other half cycle I_(bypass) flowsalternately through from drain to source of Q₁ and the diode of Q₂.

Reference is now made to FIG. 4b , which illustrates a power harvestingsystem 42 according to further embodiments. As in power harvestingsystem 41 photovoltaic inverting modules 410 a-410 n each has aphotovoltaic panel 100, bypass diode 310, control loops 404 andinverters 402 having synchronization units 408 and current bypasses 422.Modules 410 a-410 n have outputs connected in series to give a voltageoutput V_(out), which is applied to load 406. Sensor 416 preferablysenses the live voltage applied to load 406 optionally viaelectromagnetic pickup on the power line connected to load 406 ordirectly by having visibility of the grid by virtue of bypasses 422.Sensor unit 412 transfers details of the load voltage (e.g. amplitude,phase, and frequency) to synchronization unit 408 a via control line420. Control signals are optionally sent over power line communications,wireless or over a separate interface.

Although only one control line 420 is shown, optionally multiple or allsynchronization units 422 receive synchronization signals from sensor412.

Reference is now made to FIG. 5a , which shows a flow chart of a method50 illustrating operation of power harvesting systems 41 and 42according to various aspects. Method steps include installation (step500) wake-up (step 501), normal operation (step 503), and shut down(step 505).

500 Installation and 501 Wake-Up

During installation (step 500), photovoltaic modules 410 are preferablynot producing power so as not to be a safety hazard to the installers.Optionally, a “keep-alive” signal is transmitted for instance by controlunit 418 over the AC power lines. When the “keep-alive” signal is notreceived by micro-inverters 402, AC output power is disabled or notproduced. Alternatively, if the grid is “visible” to micro-inverters402, then in the absence of grid voltage, (e.g. switch 414 in FIG. 4a isopen) micro-inverters 402 do not produce AC power. Reference is now madeto FIG. 5b , which illustrates an installation method 500 according tocertain aspects. In step 500 a, input terminals of micro-inverters 402are connected to the output of photovoltaic panels 100. In step 500 b,the output terminals of photovoltaic panels 100 are connected seriallyto give a serial voltage output. After an optional predetermined timedelay (step 501 a), power inversion is enabled (step 501 b). Theenabling (step 501 b) of power inversion may be performed bysynchronization modules 408 when grid voltage is sensed or by controlunit 418 when switch 414 is closed.

503 Operation and 505 Shutdown

Reference is now made again to FIG. 5c , which shows a flow chart of amethod 503 for operating serially connected micro-inverter moduleaccording to various embodiments. Micro-inverters 402 invert (step 503b) the direct current (DC) power output of photovoltaic panels 100 toalternating current (AC) power at the outputs of micro-inverters 402while maintaining output voltage equal to the grid voltage.Synchronization (step 503 a) between the voltage outputs ofmicro-inverters 402 a-402 n and the grid voltage is maintained. Controlunit 418 optionally monitors AC synchronization between output voltageV_(out) and load 406, e.g. grid. Control unit 418 also may provideanti-islanding functionality for power harvesting system 41. If eithersynchronization and/or voltage of power harvesting system 41 isincompatible with the grid, control unit 418 disconnects powerharvesting system from the grid by signaling switch 414. Alternatively,synchronization (step 503 a) including maintenance of grid voltage isachieved using synchronization units 422 which can sense the grid byvirtue of bypass paths 422. Upon failure of either synchronization (step503 a) or inverting power at grid voltage (step 503 b) by any of theserially connected micro-inverter modules 402, then current bypassoccurs (step 503 d). Current bypass is optionally an active currentbypass using active switches as shown in FIG. 4c or preferably a passivecurrent bypass. Shutdown (step 505) occurs for instance at dusk whenlight levels are two low to maintain the grid voltage at any currentlevel. During shutdown, the photovoltaic system is optionallydisconnected from the grid using switch 414 in system 41 or in system 42each of micro inverter modules 402 stop and present high impedance tothe grid.

According to yet further embodiments, the regulation of output voltageof photovoltaic inverting modules 410 a-410 n is achieved directly bythe grid 406. The regulation does not require control unit 418 andswitch 414 as shown in FIG. 4a and relies on the fact that grid 406 isalmost infinitely greater in terms of potential supply of power bycomparison to the AC power produced by photovoltaic inverting modules410 a-410 n. The greater power of grid 06 forces photovoltaic invertingmodules 410 a-410 n to adjust to the grid voltage and as such,photovoltaic inverting modules 410 a-410 n are preferably operated togive as much voltage as possible at their outputs. Typically,photovoltaic inverting modules 410 a-410 are capable of sensing gridvoltage 406 so as to provide anti-islanding.

The definite articles “a”, “an” is used herein, such as “a photovoltaicpanel”, have the meaning of “one or more” that is “one or morephotovoltaic panels”.

Although selected embodiments have been shown and described, it is to beappreciated that changes may be made to these embodiments withoutdeparting from the principles and spirit of the invention.

1-13. (canceled)
 14. A method comprising: converting, using each of aplurality of inverters comprising output terminals connected serially toform a serial output, input power to output alternating-current (AC)power at the output terminals of each of the plurality of inverterswhile maintaining a serial voltage of the serial output substantiallyequal to and in synchronization with a grid voltage of a grid connectedacross the serial output; and in response to a failure in operating atleast one of the plurality of inverters, operating others of theplurality of inverters to adjust the serial voltage to the grid voltage.15. The method of claim 14, further comprising: disconnecting theplurality of inverters from the grid or disabling the plurality ofinverters in response to the serial voltage being less than the gridvoltage.
 16. The method of claim 15, wherein the disconnecting theplurality of inverters from the grid comprises opening a switchconnected between the plurality of inverters and the grid.
 17. Themethod of claim 14, further comprising: disconnecting the plurality ofinverters from the grid or disabling the plurality of inverters inresponse to a lack of synchronization between the serial voltage and thegrid voltage.
 18. The method of claim 14, wherein each of the pluralityof inverters comprises a control loop configured to maintain the inputpower received at a respective one of the plurality of inverters at amaximum input power.
 19. The method of claim 14, wherein the output ACpower at the output terminals of each of the plurality of inverters isat a voltage substantially less than the grid voltage.
 20. The method ofclaim 14, further comprising bypassing, in response to determining thefailure, the output terminals of the at least one of the plurality ofinverters by activating a bypass current path across the outputterminals of the at least one of the plurality of inverters.
 21. Asystem comprising: a plurality of inverters, each inverter of theplurality of inverters comprising: input terminals; and outputterminals, wherein the output terminals of the plurality of invertersare connected serially and configured to form a serial voltage outputwhen the input terminals are connected to power from a power source,wherein each inverter of the plurality of inverters is configured toconvert input power received at the input terminals of the inverter tooutput alternating-current (AC) power at the output terminals of theinverter while maintaining a serial voltage of the serial voltage outputsubstantially equal to and synchronized with a grid voltage of a gridwhen the grid is connected across the serial voltage output, and whereineach inverter of the plurality of inverters is configured to adjust theserial voltage output to the grid voltage in response to a failure inoperating another inverter of the plurality of inverters.
 22. The systemof claim 21, further comprising a control unit configured to disconnectthe plurality of inverters from the grid or disable the plurality ofinverters in response to one or both of the following conditions: theserial voltage being less than the grid voltage, or a lack ofsynchronization between the serial voltage and the grid voltage.
 23. Thesystem of claim 21, further comprising: a switch connected between theplurality of inverters and the grid; and a control unit configured toopen the switch in response to one or both of the following conditions:the serial voltage being less than the grid voltage, or a lack ofsynchronization between the serial voltage and the grid voltage.
 24. Thesystem of claim 21, further comprising: a switch connected between theplurality of inverters and the grid; and a control unit configured tosend a “keep alive” signal to the plurality of inverters, wherein theplurality of inverters are configured to convert the input power to theAC power in response to receiving the “keep alive” signal, and whereinthe control unit is configured to open the switch in response todetecting one or both of the following conditions: the serial voltagebeing less than the grid voltage, or a lack of synchronization betweenthe serial voltage and the grid voltage.
 25. The system of claim 21,wherein each of the plurality of inverters comprises a control loopconfigured to maintain the power received at the input terminals at amaximum input power.
 26. The system of claim 21, wherein each of theplurality of inverters is configured to output AC power at the outputterminals at a voltage substantially lower than the grid voltage. 27.The system of claim 21, wherein each inverter of the plurality ofinverters further comprises a bypass current path between the outputterminals of the inverter, and wherein each inverter of the plurality ofinverters is configured to bypass, in response to a failure in operatingthe inverter, the output terminals by activating the bypass currentpath.
 28. An apparatus comprising a first inverter, the first invertercomprising: input terminals; and output terminals configured to generatepart of a serial voltage output that is formed when the input terminalsare connected to input power from a power source, wherein the firstinverter is configured to convert the input power to outputalternating-current (AC) power at the output terminals of the firstinverter while maintaining, together with at least another inverter, theserial voltage output to be substantially equal to and synchronized witha grid voltage of a grid when the grid is connected across the serialvoltage output, and wherein the first inverter is configured to adjustthe serial voltage output to the grid voltage in response to a failurein operating the another inverter.
 29. The apparatus of claim 28,wherein the first inverter comprises a control loop configured tomaintain the input power received at the input terminals at a maximuminput power.
 30. The apparatus of claim 28, wherein the first inverteris configured to convert the input power to the output AC power inresponse to receiving a “keep alive” signal, and to stop converting theinput power to the output AC power in response to the “keep alive”signal not being received for a predetermined period of time.
 31. Theapparatus of claim 28, wherein the first inverter is configured to stopconverting the input power to the output AC power in response to anabsence of the grid.
 32. The apparatus of claim 28, further comprising:a switch connected between the first inverter and the grid; and acontrol unit configured to disconnect the first inverter from the gridby opening the switch in response to an islanding condition.
 33. Theapparatus of claim 28, further comprising: a switch connected betweenthe first inverter and the grid; and a control unit configured to send a“keep alive” signal to the first inverter to enable converting the inputpower to the output AC power, and to stop sending the “keep alive”signal to shut down the first inverter.
 34. The apparatus of claim 28,wherein the first inverter further comprises a bypass current pathbetween the output terminals, and the first inverter is configured tobypass the output terminals by activating the current bypass path inresponse to a failure in operating the first inverter.